The present invention relates to a central frequency estimating method for use in magnetic resonance imaging, for estimating a magnetic resonance frequency, i.e., central frequency, in certain tissue, and a magnetic resonance imaging apparatus for producing a magnetic resonance image based on the central frequency acquired by the central frequency estimating method.
Magnetic resonance imaging (MRI) is a technique of applying a gradient magnetic field and an RF (radio frequency) wave to a subject in a static magnetic field, and producing an image based on magnetic resonance signals emitted as an echo from protons in a region to be examined.
There is known a magnetic resonance imaging technique such as a CHESS (chemical selective suppression) technique, which collects magnetic resonance signals having a specific frequency suppressed, and produces an image based on the magnetic resonance signals having the specific frequency suppressed.
For example, the CHESS technique applies an RF wave having the same frequency as the resonance frequency of fat protons to a region to be examined in a subject to excite only the fat protons, and then applies a gradient magnetic field for saturating the fat protons to collect magnetic resonance signals having the resonance frequency of the fat protons suppressed (See Non-Patent Document 1, for example).
As can be seen from this, it is important to accurately know the resonance frequency of protons of tissue to be suppressed in the CHESS technique. The resonance frequency of protons of the tissue of interest is sometimes referred to as the central frequency.
In magnetic resonance imaging such as that according to the CHESS technique, it is desirable to accurately obtain the central frequency.
The resonance frequency of protons is determined by the magnitude of a static magnetic field and the gyromagnetic ratio that depends upon the type of protons in tissue. Thus, as the magnitude of a static magnetic field varies, so does the central frequency. An open magnetic resonance imaging system having a static magnetic field with magnetic field intensity around 0.2–0.7 Teslas, which is sometimes referred to as an open mid-to-low magnetic field system, for example, has a relatively inhomogeneous static magnetic field, and tends to have some positional variation in the magnitude of the static magnetic field.
Therefore, when the CHESS technique is employed in the mid-to-low magnetic field system, for example, a central frequency is desirably determined for each cross-sectional slice (referred to simply as a slice) of certain thickness that represents a region to be imaged in the subject.
The central frequency for each slice can be obtained by measurement in a process called a prescan, which is conducted before an actual scan for acquiring a magnetic resonance image of the subject.                [Non-Patent Document 1] Mugler J. P., 3rd and Brookeman Jr., “Three-dimensional Magnetization-Prepared Rapid Gradient-Echo Imaging,” Magnetic Resonance in Medicine, 15(1), pp. 152–157 (July 1990).        
When the number of slices corresponding to an imaged region is large and the central frequency is obtained for every slice, the prescan requires a long time in proportion.
If the time required for the prescan increases, the subject suffers a decline in comfort, making motion of the subject more likely, so that problems may arise, such as that reliability of data for the central frequency obtained is reduced.